Watching paint dry has never been so exciting, say scientists from the UK and France, who claim that the activity we often describe as the most boring possible is nothing of the sort. They found that during the drying process, the small particles in paint team up and come out on top – pushing the large ones to the bottom.
The researchers explained in the academic journal Physical Review Letters that they have discovered a physical mechanism that separates paint particles according to their size during the drying process.
Dr. Andrea Fortini, a post-doctoral researcher at the University of Surrey, alongside researchers at the Universities of Surrey and Claude Bernard in Lyon, France, say that their discovery may help enhance the performance of many everyday consumer goods, from paints, sunscreens to medications.
When the paint coating is first applied to a surface (left), the small (yellow) and large (red) particles are all mixed up in the solution. As the paint dries the small particles push the large ones out of their way and come out on top (right). (Image: physics.aps.org. Credit: A. Fortini/Univ. of Surrey)
Coating forms two layers spontaneously
The researchers performed computer simulations and carried out materials experiments to demonstrate how – when coatings with large and small particles such as paints dry – the coating forms in two distinct layers.
This spontaneous separation of layers could be used to control the properties of the top and bottom of paint coatings independently. This would help to significantly improve the performance of coatings in several industries, including agriculture, pharmaceuticals and beauty.
Dr. Fortini, who works in the Department of Physics at the University of Surrey, and was lead author, said:
“When coatings such as paint, ink or even outer layers on tablets are made, they work by spreading a liquid containing solid particles onto a surface, and allowing the liquid to evaporate.”
“This is nothing new, but what is exciting is that we’ve shown that during evaporation, the small particles push away the larger ones, remaining at the top surface whilst the larger are pushed to bottom. This happens naturally.”
Drawing of a wet film containing a colloidal mixture of large particles with diameter dl, and small particles with diameter ds. The film is bounded at the bottom by a substrate and at the top by the air-water interface that falls with a velocity of vev. (Image: journals.aps.org)
This type of self-layering, that occurs as the small particles make their way to the upper layer, leaving the larger particles below, could be extremely useful, the researchers believe.
Imagine a sunscreen, for example, that had small sunlight-blocking particles that pushed their way to the top, leaving the larger more adhesive particles touching the skin in the bottom layer – you would have a sunscreen that sticks to the skin better, and does a more effective job of blocking those harmful rays.
All done at microscopic level
Dr. Fortini explained:
“Typically the particles used in coatings have sizes that are 1000 times smaller than the width of a human hair so engineering these coatings takes place at a microscopic level.”
Dr. Fortini and colleagues are currently trying to find out how to control the width of the layer by changing the type and amount of small particles in the coating. They are also exploring their possible commercial usages in a wide range of industrial products, including ink, paints and adhesives.
Imagine a sun cream with adhesive (blue) and sunblocking (red) particles. After applying it to the skin, the red particles make their way to the top, while the blue ones remain at the bottom, sticking the skin.
Alexander Routh, who works at the University of Cambridge as a chemical engineering lecturer, said regarding the study:
“[The study presents] a novel, yet simple, mechanism for the observed accumulation of small particles. It opens up a whole new avenue for how small particles will arrange themselves into structures.”
In an Abstract in the journal, the authors wrote:
“During drying, the mixture stratifies into a layer of the larger particles at the bottom with a layer of the smaller particles on top. We developed a model to show that a gradient in osmotic pressure, which develops dynamically during drying, is responsible for the segregation mechanism behind stratification.”
The study is supported by the EU Project BARRIER PLUS which aims to reduce the use of environmentally harmful volatile organic compounds in paints.
Reference: “Dynamic Stratification in Drying Films of Colloidal Mixtures,” Ignacio Martín-Fabiani, Jennifer Lesage De La Haye, Pierre-Yves Dugas, Andrea Fortini, Franck D’Agosto, Elodie Bourgeat-Lami, Muriel Lansalot, Joseph L. Keddie, and Richard P. Sear. Physical Review Letters. 18 March 2016. DOI: http://dx.doi.org/10.1103/PhysRevLett.116.118301.
Video – Small particles come out on top
In this video, you can see how the small yellow particles gradually make their way to the top, resulting in two layers – the top yellow (small particles) layer and the bottom blue (large particles) one.